Democratic presidential candidate Sen. Bernie Sanders (I-VT), former Vice President Joe Biden, and Sen. Elizabeth Warren (D-MA) interact on stage during the Democratic Presidential Debate at Texas Southern University’s Health and PE Center on September 12, 2019 in Houston, Texas.Win McNamee | Getty ImagesFormer Vice President Joe Biden wasted no time in taking swings at his two biggest threats in the 2020 Democratic primary.Asked during Thursday’s debate whether Sens. Bernie Sanders, I-Vt., and Elizabeth Warren, D-Mass., had gone too far with their sweeping proposals, Biden first said “that’ll be for voters to decide.” Then he zeroed in on “Medicare for All” plans — perhaps the biggest dividing line among contenders to take on President Donald Trump next year.Flanked by Sanders and Warren at center stage, the former vice president said he was “for Barack” — tying himself to the president under whom he served and the increasingly popular Affordable Care Act he helped to pass. Then he called the single-payer health care plan that Sanders and Warren support too expensive.Biden said Warren “has not indicated how she pays for” the government-run plan. He argued Sanders “gets about halfway there.”Without criticizing each other, Warren and Sanders defended the merits of their health care proposals. Warren said “we all owe a huge debt to President Obama.” She also said that, despite likely tax increases for many families, overall “costs are going to go up” for wealthy individuals and corporations and go down for middle-class families.Sanders also called Medicare for All “the most cost-effective approach.” He again highlighted that he “wrote the damn bill” and moved the discussion forward on single-payer health care. The senator argued “we need a health care system that guarantees health care for all people.”The issue, which voters consistently list as the most important in the 2020 election, dominated more than a half hour of discussion at the start of the debate. Biden, Sanders and Warren — the three candidates who have stood out from the field in most polling — got the most speaking time on health care as they got chances to rebut other contenders’ attacks.At one point, Biden questioned Sanders, a democratic socialist, over his plan that would have companies return their health care savings to workers. The senator responded that he thought they will. Biden shot back: “For a socialist, you’ve got a lot more confidence in corporate America than I do.”Both Sen. Amy Klobuchar, D-Minn., and South Bend, Indiana, Mayor Pete Buttigieg criticized Medicare for All as going too far. Sen. Kamala Harris, D-Calif., instead said Democrats’ health care discussions were giving Americans a “headache.” She argued Democrats should focus on Trump’s past efforts to repeal Obamacare.At various points, candidates praised Obama. Former Housing and Urban Development Secretary Julian Castro used the former president — whose association with Biden has been a major part of his vice president’s campaign — to hit the Democratic frontrunner.”I am fulfilling the legacy of Barack Obama, and you’re not,” Castro, who served in Obama’s Cabinet, said.”That’ll be a surprise to him,” Biden rebutted.Subscribe to CNBC on YouTube.